The problem before was he was too safe, did that 1 second hurt a lot? Yes but it also made him fair. While I would expect him to get buffed eventually, it won't be to his ultimate, that is unfair at all that he is invuln to you, does flat damage, whatever extra damage makes it explode for me, and he could immediately poof back? That 1 second gives the enemy time to possibly take you down with him.
From the sounds of it, you were actually a good player and not just some scrub looking for freelo from an OP champ, so I will give you the benefit of the doubt, but something is indeed wrong when a crappy player could play zed and do amazing just because it is zed. I played him once in an Aram without knowing how to play him back before the nerf and I went like bloody 18/12 or something like that, sure i died a lot but for a brand new player, I almost had 20 kills since he was that tough.
Assassins should be punished if they don't delete someone. By your stat, he is 70% more likely to die, which means that you better use your ult wisely then just use it, realize you fked up and get out of jail free. It requires you to be smart. Some examples you said, Fizz and Kat can be countered too, and quite easily tbh.
Fizz you have to make him waste trickster, as soon as its on cooldown, harrass him or if he is low, go in on him. Without trickster, he has to waste flash or go all in on you and if you start the attack and he isnt fed, its more likely you will win over him. I have played games as Ziggs against him which he is meant to counter hard since Ziggs insta dies if anyone gets on him, which Fizz is very good at doing, and I would force him to waste trickster over an E if he wasnt paying attention and throw a Q where he was landing, after a few times I would go in on him, forcing him to leave or die.
Kat the problem with her isn't so much her laning, its her roaming. Kat can be countered easily just by telling where she is going in advance. She tries going bot? Tell bot in advance, not like 1 second before she is there already, and watch how she not only wasted her time and missed CS, you could have taken her tower as well. And with most Kats, they have to decide what to do with their Shunpo (I forget which slot it is atm) They either have to decide to use it offensively or with a minion or ward to get out, so try to make them waste it and its no different than Fizz, harrass them after since they have no way of getting to you without it.
Leblanc is basically same boat as Kat except she can also lane very well, but with her its basically if you have longer range, harrass her, when she W's to you, throw an ability where she placed it and she will get hit everytime, again my example is Ziggs and I always throw my E on it when she comes in, sure I may get hit hard, but she goes back to getting a lot of damage and slowed and I always land a Q on her as well, or even ULT if she is low enough, unless she can 100-0 me which isnt likely since due to being long range, I have tower protection and can push her into tower with W. All she can do then is roam and if she is too far behind, even her roam is crap, Kat has her beat that even if she is like 0/10, she can still come back, LB? if she gets into that position she is over basically.
With Zed, he just had to throw a Living Shadow down, throw a Q, switch places, Ult you, and click every button then ult back to safezone and watch as you exploded with 100% safety. So while yes, he was nerfed hard and out of play from it, but I still think his ULT should stay the way it is, if he were to get buffed, it should be to his other abilities or even his passive, he ULT should be powerful but not a powerful tool and also a get out of jail free card.